Touchscreen modules comprise touch sensitive panels superimposed over a display, so that a user can select an object (for example, text or graphics) presented on the display by touching the corresponding point on the touch sensitive panel. A touch panel controller ascertains co-ordinates of a point touched by the user and provides the co-ordinates of this point to an operating system.
A touchscreen module can be based on any one of a variety of different technologies, such as resistive, capacitive, surface acoustic wave, and the like. Capacitive sensing is becoming more prevalent and in demand for applications and there are a number of methods to measure capacitance. Some example methods are relaxation oscillator, charge time versus voltage, voltage divider, charge transfer and sigma-delta modulation. Typically, a capacitive sensing module (CSM) is utilized which produces an oscillating voltage signal, referred to as a scan signal, for measurement at a frequency dependent upon the capacitance of an object connected to the module. Typically, the CSM oscillates at a particular frequency dependent upon the capacitance of connected sensing electrodes. The CSM frequency changes when a touch is introduced near a sensing electrode because the touch introduces and changes a total capacitance presented by an electrode. The CSM frequency change is used as an indication of a touch condition.
A problem with touchscreen operation is that for compliance with regulations in Europe and other countries, a touchscreen must successfully operate when noise is injected into a system across a defined frequency range. On occasion, if a touchscreen scan signal has a scan frequency that falls within this range then the noise injected will interfere with the touchscreen operation. This may be seen as false touches and/or failure to recognize a true touch on the screen. This can lead to device failure being indicated.
In addition, in a non-testing situation, touchscreens are generally susceptible to noise interference from the surrounding environment, for example if a device is used near a transmitter. Again, if an environmental noise is introduced which is at or around a particular scan frequency utilized by a touchscreen controller, then false positives and/or device failure can occur.
Prior known systems utilize ferrites to attenuate any external noise as it enters the system. However, this can be very difficult to achieve when the conducted noise is present on the mains ground.
Prior known systems are also known that disable the touchscreen when noise is detected at a level which would interfere with operation of the device. Dis-activation of the touchscreen unfortunately often provides a disagreeable user experience.